1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, a process cartridge and an electrophotographic apparatus. More particularly, it relates to an electrophotographic photosensitive member having in its photosensitive layer a specific charge-transporting material, and a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member.
2. Related Background Art
Image-bearing members used in electrophotographic photosensitive member include electrophotographic photosensitive members. Form the viewpoints of high productivity and future developments and easiness of material designing, organic electrophotographic photosensitive members making use of organic photoconductive materials are energetically put forward. In regard to the functionality as electrophotographic photosensitive members, electrophotographic photosensitive members superior to inorganic electrophotographic photosensitive member have come to be manufactured. It, however, is desired to achieve higher sensitivity and to improve image stability in repeated use and durability in organic electrophotographic photosensitive members.
To settle these subjects, approaches to the improvement of charge-transporting materials have been proposed (see, e.g., Japanese Patent Application Laid-open Nos. H9-292724 and 2001-133994). Japanese Patent Application Laid-open Nos. H9-292724 and 2001-133994 disclose that a charge-transporting material having three or four structures having in its molecule a tertiary amine in which phenyl groups are bonded to the nitrogen atom (hereinafter “triphenylamine structures”) is used in an electrophotographic photosensitive member to achieve a higher sensitivity, and superior charge transport performance is brought out by the three or four triphenylamine structures possessed in the charge-transporting material. There, however, is no disclosure as to a charge-transporting material having five or more structures having a tertiary amine in which either of substituted or unsubstituted aromatic carbocyclic groups and substituted or unsubstituted aromatic heterocyclic groups are bonded to the nitrogen atom (hereinafter “triarylamine structures”) charge-transporting material. There is also no disclosure as to the durability in mechanical strength of the electrophotographic photosensitive member.
Similarly, Japanese Patent Application Laid-open No. 2000-206721 gives an example of a charge-transporting material having therein two to four triphenylamine structures. This Japanese Patent Application Laid-open No. 2000-206721 discloses that the charge-transporting material having two to four triphenylamine structures makes the charge transport layer have higher glass transition temperature, but admits in Comparative Examples that only the incorporation of this charge-transporting material can not achieve the improvement in durability.
Thus, the improvement of charge-transporting materials have achieved a superior charge transport performance. Since, however, a low-molecular-weight charge-transporting material is mixed in a binder resin, it would be said that the inherent mechanical strength of the binder resin is not necessarily fully exhibited.
For the purpose of more preferably preventing the layer from having a low mechanical strength because of the addition of a low-molecular-weight charge-transporting material, Japanese Patent Application Laid-open No. S61-151545 and Japanese Patent Publication No. H5-49106 disclose a high-molecular-weight charge-transporting material of a polymer type which contains triarylamine structures in a large number. In these patent publications, it is disclosed that the high-molecular-weight charge-transporting material of a polymer type which contains triarylamine structures in a large number is used in an electrophotographic photosensitive member, thereby improving its durability in repeated use. According to the disclosure in these patent publications, the high-molecular-weight charge-transporting material is synthesized by polymerization reaction, and hence it is produced in the form of a mixture containing charge-transporting materials with various molecular weights. However, in Japanese Patent Publication No. H5-49106, it is also disclosed that there is little difference from a high-molecular-weight charge-transporting material having no molecular weight distribution as long as the number of times of the repetition of repeating structural units is 10 times or less.
As examples in which a high-molecular-weight charge-transporting material of a polymer type is similarly used in an electrophotographic photosensitive member, those disclosed in International Publications No. WO00/078843 and No. WO99/32537 may be given. A high-molecular-weight charge-transporting material disclosed in International Publication No. WO00/078843 is a high-molecular-weight charge-transporting material of a polymer type having a molecular weight distribution, which is produced by polymerization reaction of a low-molecular-weight monomer, showing that the use of this charge-transporting material in the electrophotographic photosensitive member enables higher sensitivity to be achieved in virtue of an improvement in durability and an improvement in charge transport performance.
Publication No. WO00/078843 further discloses a method in which a high-molecular-weight charge-transporting material of a polymer type which contains triphenylamine structures in a large number is separated into molecular-weight fractions, and also discloses that such separation into molecular-weight fractions brings an improvement in charge transport performance. International Publication No. WO99/32537 further discloses that the use of a high-molecular-weight charge-transporting material of a polymer type brings an improvement in durability of electrophotographic photosensitive members, proposing an electrophotographic photosensitive member having a high charge transport performance and a high durability.
However, even if the polymer-type charge-transporting material has a small number of times of the repetition of repeating structural units and is separated into molecular-weight fractions, it has a molecular weight distribution and contains charge-transporting materials with various molecular weights. Hence, all the polymer-type high-molecular-weight charge-transporting materials do not necessarily have sufficient mechanical strength and electrophotographic performance. Also, even when they have a certain mechanical strength, they have such a disadvantage that their manufacturing cost is so high as to be not suited for practical use.
In addition, since an improvement in wear resistance makes the depth of wear smaller, the photosensitive layer have a longer lifetime and the influence of electrical external force coming from the steps of charging, imagewise exposure, development with toner and transfer the photosensitive layer undergoes, becomes relatively large. Hence, when the photosensitive layer is repeatedly used, such influence is liable to appear. For example, smeared images may appear in a high-humidity environment due to deterioration in the electrophotographic photosensitive member surface. Thus, problems have come to arise which should be solved at the same time the durability of electrophotographic photosensitive members is improved.